1. Field of Invention
The present invention relates to a liquid crystal device and a projection-type display device, which are suitable for homeotropic alignment.
2. Description of Related Art
A liquid crystal panel used as a liquid crystal light valve and the like is formed by sealing liquid crystal between two substrates, such as glass substrates and quartz substrates. In such a liquid crystal panel, active elements, such as thin film transistors (TFTs), and pixel electrodes connected to the active elements are arranged in a matrix on one of the substrates while a counter electrode is arranged on the other substrate. By changing the optical characteristic of the liquid crystal layer sealed between the two substrates in accordance with an image signal, the liquid crystal panel can perform a display.
In other words, the image signal can be supplied to the pixel electrodes (ITOs) arranged in a matrix by the TFT elements and the polarization state of light that passes through the pixel electrodes and the liquid crystal layer changes in accordance with the image signal. The polarization state of light that is transmitted through the liquid crystal layer changes according to the change of the arrangement of liquid crystal molecules by a voltage applied to the liquid crystal layer. A liquid crystal device is formed by arranging a polarizer on the incidence surface and the exit surface of the liquid crystal panel, respectively.
The liquid crystal device displays images in accordance with the polarization direction of light, which is caused by the polarizer, and the rotation direction of light that passes through the liquid crystal layer.
In a planer alignment where the direction of the liquid crystal molecules is set parallel to the substrate when a voltage is not applied to it, an alignment layer is formed on the surface that contacts the liquid crystal layer of one substrate (an active matrix substrate (referred to as an element substrate)) and the other substrate (a counter substrate), respectively, and subjected to a rubbing process.
Therefore, when a voltage is not applied to the liquid crystal molecules, they align in a rubbing direction. When the rubbing directions of the element substrate and the counter substrate are orthogonal to each other, the liquid crystal molecules are continuously changed their directions in the liquid crystal panel and arranged in directions different from each other by 90° between the both substrates. In this state, by providing polarizers with polarization axes orthogonal to each other on the incidence surface and the exit surface of the liquid crystal panel, respectively, to make the directions of the liquid crystal match the rubbing directions of the respective substrates, when a voltage is not applied, incident light can be rotated by 90° in accordance with the arrangement of the liquid crystal molecules in the liquid crystal layer to be emitted from a front surface of the liquid crystal panel through the polarizer. That is, in this case, a white display is performed when a voltage is not applied.
When a voltage is applied to liquid crystal, the arrangement direction of the liquid crystal changes and rotation of light in a vibration direction, which is caused by the liquid crystal in the liquid crystal panel, becomes limited. Therefore, light emitted from the front surface of the liquid crystal panel is absorbed by the polarizer. Light is transmitted in a transmittance ratio corresponding to the image signal by applying the voltage in accordance with the image signal to the liquid crystal, and thus images are displayed.
On the other hand, there is a case that a homeotropic alignment where, when voltage is not applied, the liquid crystal molecules are perpendicular to the substrates is adopted. According to the homeotropic alignment, in case where the polarization axes of the polarizers on the incidence plane of the liquid crystal panel and on the emission plane of the liquid crystal panel are orthogonal to each other, a black display is performed when voltage is not applied. The black display is performed by the two polarizers with the polarization axes orthogonal to each other. It is possible to obtain a perfect black level display in the homeotropic alignment.
However, in the liquid crystal device, deterioration of the liquid crystal, such as decomposition of a liquid crystal component, pollution by impurities generated in liquid crystal cells, and image persistanse of a displayed image occurs due to application of a direct current voltage to the liquid crystal. Therefore, in general, an inversion driving of inverting the polarity of the driving voltage of each pixel electrode at a predetermined cycle such as every frame or every field in the image signal is performed. Also, a line inversion driving method such as a 1H inversion driving method of inverting the polarity of the driving voltage in every row of the pixel electrodes at a predetermined cycle or a 1S inversion driving method of inverting the polarity of the driving voltage in every column of the pixel electrodes at a predetermined cycle is adopted.
However, when the 1H inversion driving method or the line inversion driving method is adopted, an electric field (hereinafter referred as a horizontal electric field) is generated between adjacent pixel electrodes to each other on the same substrate in a column direction or a row direction where voltages with different polarities are applied. When the horizontal electric field is generated between the adjacent pixel electrodes, the direction of the liquid crystal molecules is affected by the horizontal electric field to disorder the alignment of the liquid crystal.
In particular, in the homeotropic alignment where it is difficult to control the arrangement of the liquid crystal molecules when voltage is not applied and the rotation of the liquid crystal molecules when the voltage is applied, the liquid crystal molecules are significantly affected by the horizontal electric field to easily disorder the alignment. Therefore, in the liquid crystal device using the homeotropic alignment, a method in which a step is provided in the liquid crystal layer in order to define the alignment of the liquid crystal molecules is adapted sometimes.
However, in a projection-type display device using the liquid crystal panel as a light valve, there are problems that the area of the substrate of the liquid crystal panel is small, and an aperture ratio is significantly lowered when the step is provided.
Therefore, as the liquid crystal device using the homeotropic alignment where it is difficult to define the alignment direction of the liquid crystal molecules, one which employs circular polarized light as transmission light has been developed. The circular polarized light is an isotropic polarization in which does not have deflection in a polarization direction, and thus can achieve a high contrast image without damaging brightness. Japanese Unexamined Patent Application Publication No. 2002-40428 describes about the circular polarized light.